Revert^2 "Move GCStress BacktraceHelper to libbacktrace"

By changing to keeping the BacktraceMap in each threads TLS we avoid
having to recreate it each time we do an allocation in GCStress. This
makes running in gcstress significantly faster than it was in the
initial version of this CL.

For test 001-HelloWorld, this has an ~30% measured time cost as
compared to libgcc (from 9.063s to 12.262s). On the other hand it
seems better at actually differentiating distinct stack traces. Total
time spent doing gcs is about doubled (from 4107.91ms to 7036.97ms).
Total number of gcs is about doubled (from 175 to 320).

This reverts commit 7d3256a3.

Reason for revert: Fixed issue causing gcstress to take too long.

Test: ./test/testrunner/testrunner.py --host --gcstress --jit --64 -t 989
Test: ./test/testrunner/testrunner.py --host --gcstress

Change-Id: I668f53513d8495d4a4c7aa540137676118903f8a